Orthorhombic Deformation Research Articles

Long-range magnetic order in CePdAl3 enabled by orthorhombic deformation

We investigate the effect of structural deformation on the magnetic properties of orthorhombic CePdAl3 in relation to its tetragonal polymorph. Utilizing x-ray and neutron diffraction, we establish that the crystal structure has the Cmcm space-group symmetry and exhibits pseudotetragonal twinning. According to density functional calculations, the tetragonal-orthorhombic deformation mechanism has its grounds in the relatively small free enthalpy difference between the polymorphs, allowing either phase to be quenched, and fully accounts for the twinned microstructure of the orthorhombic phase. Neutron diffraction measurements show that orthorhombic CePdAl3 establishes long-range magnetic order below TN=5.29(5) K characterized by a collinear, antiferromagnetic arrangement of magnetic moments. Magnetic anisotropies of orthorhombic CePdAl3 arise from strong spin-orbit coupling as evidenced by the crystal-field splitting of the 4f multiplet, fully characterised with neutron spectroscopy. We discuss the potential mechanism of frustration posed by antiferromagnetic interactions between nearest neighbors in the tetragonal phase, which hinders the formation of long-range magnetic order in tetragonal CePdAl3. We propose that orthorhombic deformation releases the frustration and allows for long-range magnetic order. Published by the American Physical Society 2024

Strain-Induced Ferromagnetic to Antiferromagnetic Crossover in d9-Ion (Cu2+ and Ag2+)-Layered Perovskites

A characteristic aspect of undoped high-temperature layered copper oxide superconductors is their strong in-plane antiferromagnetic coupling. This state is markedly different from that found in other chemically similar copper- or silver-layered fluorides, which display a ferromagnetic ground state. The latter has been connected in the literature with the presence of an orthorhombic deformation of the lattice that shifts the intermediate ligand between two metal ions to be closer to one and further from the other. This distortion is completely absent in the oxides, which are essentially tetragonal. However, no quantitative information exists about how this distortion influences the antiferromagnetic state and its relative stability with respect to the ferromagnetic state. Here, we carry out first-principles simulations to show that the fluorides in the parent tetragonal phase are also antiferromagnetic and that the antiferromagnetic-to-ferromagnetic transition is only triggered for a large enough distortion, with a typical ligand shift of 0.1 Å. Moreover, we employ a valence-bond model and second-principles simulations to show that the factor in superexchange that favors the antiferromagnetic state reduces as the ligand moves away from the symmetric metal–metal position. Importantly, we find that this distortion is sensitive to the application of an epitaxial strain which, in turn, allows controlling the difference of energy between ferromagnetic and antiferromagnetic states and thus the Curie or Néel temperatures. In fact, for compressive strains larger than 5.1%, this piezomagnetic effect makes K2CuF4 and Cs2AgF4 antiferromagnetic, making these two lattices close chemical analogs of oxide superconductors.

InBi: A Ferroelastic Monolayer with Strain Tunable Spin-Orbit Dirac Points and Carrier Self-Doping Effect.

Semimetallic two-dimensional (2D) Dirac materials beyond graphene, especially 2D materials with robust Dirac points against the spin-orbit coupling (SOC), are still highly sought. Herein, we theoretically demonstrate the InBi monolayer as a long-sought 2D Dirac material whose exotic Dirac Fermionic states cannot be gapped out by SOC. The InBi monolayer with the litharge crystal structure possesses not only 4-fold band degeneracy, linear energy dispersion, and ultrahigh Fermi velocity in the order of 105 m/s, but also spontaneous ferroelasticity that can lead to the orthorhombic lattice deformation and semimetallic electronic structure. Specifically, the symmetry protected spin-orbit Dirac points in 2D InBi are located at the Brillouin Zone (BZ) boundary and near the Fermi level in energy. More importantly, with coexisting spin-orbit Dirac points and spontaneous ferroelasticity, the InBi monolayer exhibits an additional advantage for engineering Dirac Fermionic states by ferroelastic (FE) strain. Energy levels of Dirac points are strongly coupled to FE strain, and the semimetallic electronic structure of the InBi monolayer is also susceptible to the FE strain induced carrier self-doping effect. Depending on the strain orientation within the InBi monolayer, electron and hole Fermi pockets will develop along the two planar directions, leading to the characteristic transport coefficients (as evidenced by our transport simulations based on Boltzmann formalism) for future experimental detection. FE strain tunable Dirac Fermionic states together with the carrier self-doping effect will benefit future development of ultrathin electronic devices with both high carrier mobility and controllable charge conductivities.

Structural Phase Transitions in closo-Dicarbadodecaboranes C2B10H12.

The crystal structures of three thermal polymorphs (I, II, and III) for each isomer of closo-dicarbadodecaboranes C2B10H12 (ortho, meta, and para) have been determined by combining synchrotron radiation X-ray powder diffraction and density functional theory calculations. The structures are in agreement with previous calorimetric and spectroscopic studies. The difference between rotatory phases (plastic crystals) I and II lies in isotropic rotations in the former and anisotropic rotations of the icosahedral clusters in the latter. Phase I is the cubic close packing (ccp) of rotating closo-molecules C2B10H12 in the space group Fm3̅. Phase II is the ccp of rotating closo-molecules C2B10H12 in the cubic space group Pa3̅. The preferred rotational axis in II varies with the isomer. The ordered phases III are orthorhombic (meta) or monoclinic (ortho and para) deformations of the cubic unit cell of the disordered phases I and II. The ordering in the phase III of the ortho-isomer carrying the biggest electrical dipole moment creates a twofold superstructure w.r.t. the cubic unit cell. The thermal polymorphism for C2B10H12 and related metal salts can be explained by division of the cohesive intercluster interactions into two categories (i) dispersive cohesive interaction with additional Coulombic components in the metal salts and (ii) anisotropic local interaction resulting from nonuniform charge distribution around icosahedral clusters. The local interactions are averaged out by thermally activated cluster dynamics (rotations and rotational jumps) which effectively increase the symmetry of the cluster. The C2B10H12 molecules resist at least as well as the CB11H12– anion to the oxidation, and both clusters form easily a mixed compound. This allows designing solid electrolytes such as Nax(CB11H12)x(C2B10H12)1–x, where the cation content may be varied and the temperature of transition into the disordered conducting phase is decreased.

Half-metallic properties of Zr2CrAl ferrimagnetic full-Heusler compound, investigated in tetragonal, orthorhombic and rhombohedral crystal structures

The research efforts focused on the implementation of Heusler materials into spintronic heterostructures triggered the investigations on the half-metallic and magnetic properties on Zr2CrAl ferrimagnetic compound. The theoretical investigations based on Density Functional Theory were performed on the tetragonal, orthorhombic and rhombohedral distortions of the typical inverse-Heusler cubic crystalline structure, previously reported. The compound exhibits half-metallic characteristics for all deformations considered. The slight variations in volume of the unit cell do not destroy the high spin polarization state. The band gap from minority spin channel slightly decreases in orthorhombic and rhombohedral deformations. The total magnetic moment calculated per formula unit is integer (equal to 1μB/f.u.) and follows the Slater-Pauling rule for all cases investigated. The antiferromagnetic coupling between the Zr and Cr atoms was preserved. The functionality of ferrimagnetic full-Heusler alloys to be used as overlayers in the production of epitaxially deposited heterostructures on substrates with crystalline structures other than cubic was proven.

A new deformation family of Schwarz’ D surface

We prove the existence of a new 2-parameter family o$\Delta$ of embedded triply periodic minimal surfaces of genus 3. The new surfaces share many properties with classical orthorhombic deformations of Schwarz' D surface, but also exotic in many ways. In particular, they do not belong to Meeks' five-dimensional family. Nevertheless, o$\Delta$ meets classical deformations in a 1-parameter family on its boundary.

Two-dimensional ferroelasticity and ferroelastic strain controllable anisotropic transport properties in CuTe monolayer.

Two-dimensional ferroelastic (2D-FE) materials where FE strain originates from the lattice deformation associated with spontaneous FE phase transition, hold great promise as miniaturized shape-memory devices. Moreover, the structural anisotropy within the low-symmetry 2D-FE materials can usually lead to intrinsic anisotropy in their electronic or transport properties as well. As a result, the strong coupling of FE strain with the anisotropic electronic structure or electric-/thermoelectric-transport will largely extend the functionality and device applications for 2D-FE materials. In the current work, after performing comprehensive first-principles calculations in combination with transport simulations based on the Boltzmann formalism, we identify the experimentally synthesizable CuTe monolayer as a new 2D-FE material whose anisotropic electric- and thermoelectric-transport properties can be effectively manipulated by FE strain. Typically, CuTe monolayers that can be potentially exfoliated from the synthesized van der Waals (vdW) layered CuTe bulk are predicted to exhibit the room temperature stable ferroelasticity and large axial FE strain (up to 18.4%) created by the in-plane orthorhombic lattice deformation. Owing to the planar orientation dependent metallic vs. nearly semiconducting electronic structure, highly anisotropic electric conductivity and thermopower coefficient can be obtained along the two planar principal axes of the CuTe monolayer. To simulate the more realistic experimental scenarios, coherent formation of FE domain walls and domain-wall motion assisted FE switching have also been evaluated in CuTe multi-domain configurations. Based on the transverse thermoelectric effect inherent in anisotropic CuTe monolayers, the schematic model for obtaining the FE strain controllable electric current within CuTe multi-domain configurations has been proposed, which can be verified experimentally.

Pseudo-Jahn-Teller Effect and Magnetoelastic Coupling in Spin-Orbit Mott Insulators.

The consequences of the Jahn-Teller (JT) orbital-lattice coupling for magnetism of pseudospin J_{eff}=1/2 and J_{eff}=0 compounds are addressed. In the former case, represented by Sr_{2}IrO_{4}, this coupling generates, through the so-called pseudo-JT effect, orthorhombic deformations of a crystal concomitant with magnetic ordering. The orthorhombicity axis is tied to the magnetization and rotates with it under magnetic field. The theory resolves a number of puzzles in Sr_{2}IrO_{4} such as the origin of in-plane magnetic anisotropy and magnon gaps, metamagnetic transition, etc. In J_{eff}=0 systems, the pseudo-JT effect leads to spin-nematic transition well above magnetic ordering, which may explain the origin of "orbital order" in Ca_{2}RuO_{4}.

Structural deformation of the S=1 kagome-lattice compound KV3Ge2O9

The dielectric and structural properties of the $S=1$ kagome antiferromagnet ${\mathrm{KV}}_{3}{\mathrm{Ge}}_{2}{\mathrm{O}}_{9}$ are examined. The low-temperature structure below 50 K is orthorhombic with a typical correlation length of 8 nm. While the high-temperature hexagonal phase can be considered $C$-centered orthorhombic, the $C$-centered symmetry is broken below 50 K. The low-temperature symmetry does not support the simplex solid state, which is the theoretically expected ground state for the $S=1$ kagome lattice. Above 60 K, incommensurate lattice modulation is observed. The lock-in transition suggests that the origin of the orthorhombic deformation is the development of a short-range magnetic ordering.

Comprehensive study on effect of electron beam irradiation on electrical, thermo-electric and magnetic properties of oxygen rich LaMnO3.15 compound

We report the influence of electron beam (EB) irradiation on the structural, electrical, thermoelectric and magnetic properties of oxygen rich LaMnO3 manganite. Refinement of XRD data shows that pristine as well irradiated samples are single phased and exhibit orthorhombic distorted structure (Pbnm) with the orthorhombic deformation slightly increasing with EB dosage. The MnO bond length increases with increase in EB dosage perhaps due to strain induced by EB irradiation. On contrary, the MnOMn bond angle remains unchanged for the irradiated samples. Electrical resistivity results shows that the small polaron hopping model is valid in the high temperature regime for all the samples. The pristine sample shows a small metal-insulator transition region around the characteristic temperature TMI, however EB irradiation induces a sharper metal-insulator transition behavior. The temperature dependence of Seebeck coefficient (S) of the pristine sample displays a crossover from positive to negative value as the sample is cooled whereas the irradiated samples exhibit only positive values. The Seebeck coefficient data is consistent with the resistivity data in the sense that the SPH model is valid in high temperature region. The magnetic measurements confirm the existence of short-range FM interaction above TC and all the samples follow Curie-Weiss law in the paramagnetic state.

First-Principles Studies on d 0 Magnetism in Zinc-Blende IV-IV Compounds-Based Short-Period Heterostructures (SiC)1/(KC)1, (GeC)1/(KC)1, (SiC)1/(CaC)1, and (GeC)1/(CaC)1

With extensive first-principle calculations based on density functional theory, short-period heterostructures based on zinc-blende SiC and GeC exhibiting d 0 magnetism were modeled and simulated. The results implied that the heterostructures should be orbital-resolved hole-induced magnetic materials, in which the magnetism is driven by 2p partially filled states. Our results showed the similarities and the differences in electronic structures (electron occupation) and magnetic properties (spin configuration) between the XC (X = K and Ca) and (MC)1/(XC)1 (001) (M = Si and Ge; X = K and Ca). By adding Coulomb force, the metal to half-metal and half-metal to semiconductor transitions were found in K- and Ca-embedded heterostructures, respectively. Moreover, the results confirmed that the coexistence of covalency and half metallicity was critical for the existence of the spin-resolved pseudogap and the pseudogap formation was important for the real forbidden gap opening. The similarities and differences of electronic structures and magnetic properties between (MC)1/(KC)1 and (MC)1/(CaC)1 were analyzed by lattice symmetry and orbital occupation. In addition, to predict the structural stability, these heterostructures were computed upon tetragonal, orthorhombic, and rhombohedral deformations and these compounds are found to be energetically stable against these three lattice deformations.

Influence of electron beam irradiation on electrical, structural, magnetic and thermal properties of Pr0.8Sr0.2MnO3 manganites

In this communication, the effect of electron beam (EB) irradiation on the structural, electrical transport and thermal properties of Pr0.8Sr0.2MnO3 manganites has been investigated. Rietveld refinement of XRD data reveals that all samples are single phased with orthorhombic distorted structure (Pbnm). It is observed that the orthorhombic deformation increases with EB dosage. The Mn–O–Mn bond angle is found to increase with increase in EB dosage, presumably due to strain induced by these irradiations. Analysis on the measured electrical resistivity data indicates that the small polaron hopping model is operative in the high temperature region for pristine as well as EB irradiated samples. The electrical resistivity in the entire temperature region has been successfully fitted with the phenomenological percolation model which is based on phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. The Seebeck coefficient (S) of the pristine as well as the irradiated samples exhibit positive values, indicating that holes is the dominant charge carriers. The analysis of Seebeck coefficient data confirms that the small polaron hopping mechanism governs the thermoelectric transport in the high temperature region. In addition, Seebeck coefficient data also is well fitted with the phenomenological percolation model. The behavior in thermal conductivity at the transition is ascribed to the local anharmonic distortions associated with small polarons. Specific heat measurement indicates that electron beam irradiation enhances the magnetic inhomogeneity of the system.

Close correlation between magnetic properties and the soft phonon mode of the structural transition inBaFe2As2andSrFe2As2

Parent compounds of Fe-based superconductors undergo a structural phase transition from a tetragonal to an orthorhombic structure. We investigated the temperature dependence of the frequencies of transverse acoustic (TA) phonons that extrapolate to the shear vibrational mode at the zone center, which corresponds to the orthorhombic deformation of the crystal structure at low temperatures in BaFe 2 As 2 and SrFe 2 As 2 . We found that acoustic phonons at small wavevectors soften gradually towards the transition from high temperatures, tracking the increase of the size of slowly fluctuating magnetic domains. On cooling below the transition to base temperature the phonons harden, following the square of the magnetic moment. Our results provide evidence for close correlation between magnetic and phonon properties in Fe-based superconductors.

Effect of Co substitution on magnetic ground state in Sm0.5Ca0.5MnO3

The effect of cobalt doping on the structural and magnetic properties of polycrystalline Sm0.5Ca0.5Mn1−xCoxO3 (0⩽x⩽0.20) has been studied. Powder X-ray diffraction (XRD), dc magnetization and ac susceptibility measurements have been performed. The orthorhombic deformation caused by the Jahn–Teller (JT) effect can be counterbalanced by Co3+ doping. Meanwhile, antiferromagnetic (AFM), spin glass (SG) and phase separation (PS) ferrimagnetic states appear, successively, with the increase of x. The critical slowing down model and the dynamic scaling equation were applied in some samples. The existence of two distinct length scales for SG and finite FM (ferromagnetic) order is the source of the progressive deviation which exists in the dynamic scaling fitting for x=0.05. Moreover, the small peaks at ∼8K in χ″(T) curves are believed to be induced by the canted moments of Mn ions as observed in Nd2/3Ca1/3MnO3.

Analytic bond-order potentials for the bcc refractory metals Nb, Ta, Mo and W

Bond-order potentials (BOPs) are based on the tight-binding approximation for determining the energy of a system of interacting atoms. The bond energy and forces are computed analytically within the formalism of the analytic BOPs. Here we present parametrizations of the analytic BOPs for the bcc refractory metals Nb, Ta, Mo and W. The parametrizations are optimized for the equilibrium bcc structure and tested for atomic environments far from equilibrium that had not been included in the fitting procedure. These tests include structural energy differences for competing crystal structures; tetragonal, trigonal, hexagonal and orthorhombic deformation paths; formation energies of point defects as well as phonon dispersion relations. Our tests show good agreement with available experimental and theoretical data. In practice, we obtain the energetic ordering of vacancy, [1 1 1], [1 1 0], and [1 0 0] self-interstitial atom in agreement with density functional theory calculations.

Comparison of analytic and numerical bond-order potentials for W and Mo

Bond-order potentials (BOPs) are derived from the tight-binding approximation and provide a linearly-scaling computation of the energy and forces for a system of interacting atoms. While the numerical BOPs involve the numerical integration of the response (Green’s) function, the expressions for the energy and interatomic forces are analytical within the formalism of the analytic BOPs. In this paper we present a detailed comparison of numerical and analytic BOPs. We use established parametrizations for the bcc refractory metals W and Mo and test structural energy differences; tetragonal, trigonal, hexagonal and orthorhombic deformation paths; formation energies of point defects as well as phonon dispersion relations. We find that the numerical and analytic BOPs generally are in very good agreement for the calculation of energies. Different from the numerical BOPs, the forces in the analytic BOPs correspond exactly to the negative gradients of the energy. This makes it possible to use the analytic BOPs in dynamical simulations and leads to improved predictions of defect energies and phonons as compared to the numerical BOPs.

Effect of inhomogeneous deformation on the electronic structure of SnO2 and Sn x Sb1–x O2 phases

The electronic structures of various phases in the Sn–Sb–O2 system under pressure and under tetragonal, monoclinic, and orthorhombic deformation are studied. Calculations are performed using the first-principles pseudopotential method. It is established that SnO2 undergoes the following phase transitions under pressure: rutile–pyrite (17 GPa) and pyrite–fluorite (138 GPa). It is also found that doping SnO2 with Sb leads to a shift of the Fermi level to the conduction band and to additional resonant states below the valence band. Inhomogeneous deformation of Sn x Sb1–x O2, x = 1.00; 0.94; 0.88, at δ ≤ 0.2 causes a stress up to 6.2 GPa, depending on the strain. An analysis of the density of electron states in the bandgap of the deformed structures shows that the gap broadens under tetragonal deformation and narrows under orthorhombic and monoclinic deformation. The theoretical results obtained should be taken into account in interpreting the piezoresistive properties of Sn–Sb–O2-based thick films.

Low-temperature crystal structure and magnetic properties of Gd5Ge3

The Gd${}_{5}$Ge${}_{3}$ compound prepared using high-purity gadolinium undergoes a structural deformation from hexagonal Mn${}_{5}$Si${}_{3}$-type structure into a new orthorhombic crystal structure. The temperature of the structural transition coincides with the Ne\'el temperature (${T}_{\mathrm{N}}$), confirming the coupling between magnetism and crystal structure. The same intermetallic compound prepared using a commercial grade of gadolinium shows only a minor orthorhombic structural deformation at ${T}_{\mathrm{N}}$, which is 19 K lower than that of a Gd${}_{5}$Ge${}_{3}$ sample prepared from the high-purity Gd. The first-principles electronic structure calculations confirm the antiferromagnetic orthorhombic structure as the ground state of Gd${}_{5}$Ge${}_{3}$ and show that interstitial impurities affect the strength of the antiferromagnetic interactions.

Response of fcc metals and L12 and D022 type trialuminides to uniaxial loading along [100] and [001]: ab initio DFT calculations

Ab initio density-functional calculations have been used to investigate the response of the face-centred cubic (fcc) metals Al and Cu, and of the L12- and D022-type trialuminides Al3(Sc,Ti,V) to uniaxial loading along the [100] and [001] directions. The results obtained under uniaxial strains are compared to the response to biaxial (epitaxial) strains. The ideal tensile and compressive strengths and their limitation by shear instabilities along these deformation paths have been calculated. Although the response of both pure fcc metals could be expected to be very similar, our results show a fundamental difference: whereas for Cu a special invariant state with C 22 = C 23, leading to a bifurcation from the tetragonal to an orthorhombic deformation path, is reached at a strain of 10%, for Al this state is reached only at a strain of 33% close to the critical strain defining the ideal tensile strength. The reaction of the L12-type trialuminides is comparable to the response of Al; no bifurcation to an orthorhombic deformation path is predicted. The response of the D022-type trialuminides is different from that of the L12-type phases because of the difference in the stacking of the atomic planes along the [001] direction. For D022-type trialuminides, the uniaxial compression along this direction or epitaxial tension in the (001) plane leads to the formation of a stress-free D03 structure, in complete analogy to the fcc ⇆ bcc transformations observed for the pure metals. Under uniaxial [100] loading the guiding symmetry along the deformation path is orthorhombic and leads to the formation of special structures under both tension and compression parts, which are related to the D03 structure in the same way as the parent D022-lattice is related to the L12 structure.

First-Principles Study of the γ Angle Deformation Path in the Wurtzite-to-Rocksalt Phase Transition in Aluminum Nitride

A new transition path (γ angle deformation path) is put forward and used to characterize the wurtzite-rocksalt transition in A1N. The enthalpy surface and the contour plot of enthalpy difference at equilibrium pressure are obtained by first-principles pseudopotential method within the generalized gradient approximation. The phase transition is needed to overcome two barriers and a metaphase arises between them. The total barrier height is 0.26 eV. The pressure region of the phase transition is estimated to be 13.9–23.5 GPa, in which the experimental result is well located. Along the least barrier path, the changes of density of states and the fields of charge density difference are investigated. The similarities and differences between γ angle deformation path and the orthorhombic deformation path are analyzed.